The invention refers to an illumination device for a DUV microscope. Furthermore, the invention refers to a DUV microscope having an illumination device.
Illumination devices for DUV microscopes must make available a narrow DUV wavelength band from the DUV wavelength region (DUV=deep ultraviolet, approx. 200 to 300 nm) for the illumination for which the microscope optical system is corrected. The DUV wavelength band is characterized by the spectral peak value position and the half-value width of its peak. Both, narrow band transmission filter systems and reflection filter systems, are known for generation of a DUV wavelength band of this kind. These filter systems are inserted into the illumination beam path and filter out of the light spectrum of a light source a desired DUV wavelength band as the usable light. The microscope image is visualized with a DUV-sensitive TV camera.
Narrow-band transmission filter systems in the DUV yield very narrow peaks, but the maximum transmission and thus the maximum value of the peak is only approximately 20% of the light intensity before to the narrow-band transmission filter system. Such weak intensities, however, require particularly sensitive and therefore technologically complex and very expensive cameras. The essential disadvantage of these cameras, however (for example, in the case of so-called xe2x80x9cframe transfer CCD imagersxe2x80x9d), is that they have very long exposure and image readout times, so that xe2x80x9clivexe2x80x9d observation of the object is impossible. This is particularly problematic if the object stage with the object upon it needs to be shifted.
Known reflection filter systems comprise a plurality of reflective filters at each of which the light from the light source arrives and is reflected at an angle of incidence (xcex1=45xc2x0. With these known reflection filter systems, the maximum value of the peak is well over 90%. Their disadvantage, however, is the fact that their half-value width is approximately 30 to 50 nm. It is, however, extremely difficult and complex to correct microscope objectives with a short focal length, i.e. high magnification, in the DUV wavelength region for such a broad DUV wavelength band.
It is therefore the object of the invention to describe an illumination device for a DUV microscope which makes available a DUV wavelength band with maximum transmission and a narrow half-value width.
This object is achieved by an illumination as claimed in independent Claim 1. Advantageous embodiments of the illumination device are recited in the dependent claims.
It is a further object of the invention to provide an illumination device for a DUV microscope which allows switching between at least two different illumination wavelength bands. Furthermore, one of the wavelength bands is a DUV wavelength band having maximum transmission and a narrow half-value width.
This object is achieved by an illumination device as claimed in independent Claim 10. Advantageous embodiments of the illumination device are recited in the dependent claims.
It is another object of the invention to provide a DUV microscope with an illumination device which makes available a DUV wavelength band with maximum transmission and a narrow half-value width and which allows switching between at least two different illumination wavelength bands.
This object is achieved by a DUV microscope with an illumination device as claimed in independent Claim 25. Advantageous embodiments of the DUV microscope are recited in the dependent claims.
The invention is based on the idea that the large half-value widths are caused essentially by polarization effects in the reflection filter systems. These are much less for smaller reflection angles. A reflection filter system having small reflection angles and spectrally adapted reflection filters is therefore used in a DUV illumination system according to the present invention.
An illumination system according to the present invention for a DUV microscope has a light source from which an illumination beam path proceeds. Arranged in the illumination beam path are a condenser and a reflection filter system for generating a desired DUV wavelength band. The reflection filter system comprises four reflection filters at which the illumination beam is reflected, at the same reflection angle xcex1 in each case. The illumination beam extends coaxially in front of and behind the reflection filter system. According to the present invention, the reflection angle xcex1 at the individual reflection filters is xcex1xe2x89xa630xc2x0.
The reflection filter system supplies a very narrow DUV wavelength band xcexDUV+xcex94xcex with a half-value width of max. 20 nm. It has a peak with a maximum value S of more than 90% of the incoming light intensity. Depending on the reflection filters used, a maximum value S of more than 98% of the incoming light intensity can be achieved with a reflection filter system using small angles of incidence. For that purpose, reflection filters matched specifically to the desired DUV wavelength band xcexDUV+xcex94xcex are used in the reflection filter system of the illumination device according to the present invention.
As a result of the transition to the small reflection angles xcex1=30xc2x0 according to the present invention in the reflection filter system, polarization effects at the reflection filters can be reduced and narrow half-value widths of this kind can be achieved. Appropriately corrected DUV microscope objectives can be computed for half-value widths of the desired order of magnitude. At the same time, a DUV microscope having an illumination device according to the present invention possesses sufficiently high intensities in the illumination beam to dispense with complex special DUV-sensitive cameras, for example the aforesaid frame transfer CCD cameras with their slow image buildup.
The subject matter of the invention is described in more detail with reference to the embodiments shown in the schematic drawings,